The reproduction of electronic video images involves the transmission of a waveform known as a video signal. Various types of video signals, such as for example composite video, S-video, and component video, have certain shared characteristics. In general, the video signal includes both video image display information and associated synchronizing information. A graphical representation of a generic video signal 99 known in the arts is shown in FIG. 1. As shown in the example, the video information is generally in the form of a signal having a black reference level 100, and a higher level representing white 200. The continuum of levels 300 between the black level 100 and the white level 200 is then used to represent degrees of gray. The synchronizing information 400 used in formatting the signal display includes vertical and horizontal display alignment and color decoding information. The lowest value of the synchronizing signal is referred to as the “sync tip” 500. The synchronizing pulses 400 are positioned in portions of the signal 99 that do not contain video display information, that is, below the black level 100 to prevent disruption of the video image display. This is referred to as the “blank level” 600. The synchronizing pulses 400 shown in this example have a reference level at the blank level 600.
The video signal waveform 99 from the sync tip level 500 to the white level 200 used in this example has a nominal peak-to-peak amplitude of 1V. The “front porch” 900 refers to the portion of the video signal 99 that occurs between the end of the active video interval 800 and the falling/leading edge of the horizontal sync pulse 500. The “back porch” 700 refers to the portion of the video signal 99 that occurs between the rising/trailing edge of the horizontal sync pulse 500 and the beginning of the active video interval 800.
Automatic gain control (AGC) circuits are useful for many applications including the display of video images. Stored information concerning the video signal may have been recorded at different recording levels, or variations in the amplitude of the incoming video signal may occur for other reasons. AGC circuits are used to provide an output of constant amplitude from an input varying in amplitude. Constant output is achieved by providing a gain in inverse proportion to the input amplitude.
Although video signals in the most commonly used formats, NTSC, PAL and SECAM, for example, are analog, they are often encoded and decoded digitally. The amplitude of the video signal is often controlled utilizing a two-stage automatic gain control (AGC) scheme where a coarse gain adjustment is performed in the analog domain prior to analog-to-digital conversion and a fine gain adjustment is performed in the digital domain after analog-to-digital conversion. The overall system gain (excluding the gain of the analog-to-digital converter) is equal to the product of the coarse analog gain and the fine digital gain. Problems are encountered in the arts whenever the two-stage AGC must change the coarse gain to maintain the proper video signal amplitude. If the upper limit of the fine gain control range is reached (overflow condition), the coarse gain must be increased; if lower limit of the fine gain control range is reached (underflow condition), the coarse gain must be decreased. Ideally, the coarse gain stage would have exponential gain steps (e.g. 2 dB/step) equal to the full-scale control range of the fine gain stage (e.g. 2559/2048). However, the coarse gain stage often has linear gain steps since it is less expensive to implement in silicon. This causes the product of the coarse gain and the fine gain to be non-monotonic which leads to undesirable video amplitude fluctuations whenever the fine digital gain overflows or underflows into the coarse analog gain. If the AGC updates the gain at a line rate, these amplitude fluctuations might not be noticeable (i.e. detectable by the human eye); however, if the AGC updates the gain at a slower rate, such as the frame rate, amplitude fluctuations typically cause easily visible flashing, also known as frame flicker. Various technical advantages may be obtained by updating the gain at a frame rate, however. It would be useful and desirable in the arts to provide automatic gain control methods and systems with the capability of reaching an optimum gain setting at a frame rate without detriment to the video image. It would also be advantageous for such AGC methods and systems to converge quickly to an approximately optimal gain value.